Various products containing the naturally-occurring mineral, gypsum, have been developed for the building trades. Especially notable are various gypsum wall board products, e.g., sheet materials typically faced with paper. Desirable characteristics of such products include their strength at relatively low density (about 0.7 gm/cm.sup.3), ease of handling and fabrication, and low cost. In producing gypsum building products, calcined gypsum, i.e., anhydrous or hemihydrate (CaSO.sub.4.1/2H.sub.2 O), known also as stucco, plaster of paris, molding plaster, building plaster, and the like, typically in an aqueous slurry, is cast, molded, and dried. During the course of this process, the calcined gypsum is further hydrated, yielding CaSO.sub.4.2H.sub.2 O.
Although the hydration adds only about 20% to the weight of the calcined gypsum, excess water generally is present in the slurry to decrease the viscosity and facilitate molding. However, the excess water is undesirable in other respects. For example, it must be removed in subsequent processing of the product, at considerable cost. Furthermore, the resultant dried product is of low density and compressive strength. Consequently, it is conventional wisdom in the art that the amount of water should be held to a minimum, and various additives and treatments have been proposed to fluidize the slurry, but minimize the water; e.g., U.S. Pat. Nos. 2,913,308; 4,222,984; and 4,252,568. "Dry" or "semi-dry" processes have been described in which water required for the hydration chemistry is supplied by a water-saturable filler, such as wood 'shavings, wood fiber granulate and bark; e.g., U.S. Pat. No. 4,328,178. In this regard, the article entitled "Inorganically Bonded Wood" by A. A. Moslemi, Chem. Tech., August 1988, pp 504-510, summarizes the state of the art.
A number of building materials have been disclosed in which substantial quantities of cellulosic fillers, including wood particles and fibers, have been included in gypsum-containing products, not only as aids to incorporating the water necessary to hydrate the gypsum, but also to strengthen and otherwise upgrade the mechanical properties of the product. For example, U.S. Pat. No. 3,951,735 discloses a strengthened gypsum wallboard material having a density of 1.1-1.6 gm/cm.sup.3 (obtained by compressing a lamination) which includes calcined gypsum and cellulosic fiber such as paper pulp, but the criticality of also including a substantial amount of asbestos fiber is pointed out. Similarly, U.S. Pat. No. 4,127,628 discloses a multi-layered gypsum product of low density (0.3-0.9 gm/cm.sup.3) which includes glass fibers and optionally also contains pulp and polyvinyl alcohol, but, in addition, a substantial amount of asbestos fiber. Products which require asbestos to attain superior properties are difficult to justify in view of the environmental and health hazards associated with such products.
U.S. Pat. No. 4,239,716 describes a gypsum-containing building product containing a reinforcing agent which may be wood pulp or glass fibers, together with a binder, such as polyvinyl acetate. However, the disclosure is limited to the use of fibrous alpha-calcium sulfate hemihydrate, a very expensive raw material, requiring special conditions to produce, which is difficult to reconcile if equivalent properties could be obtained in a product which employs common and inexpensive non-fibrous forms of calcined gypsum.
Gypsum building materials generally are held in high regard for use in fire-resistant construction. The spread of fire and the penetration of flame through set gypsum structures is delayed, because impinging heat initially operates to reverse the hydration reaction, recalcining the gypsum, liberating water. The liberated water is an additional energy sink, absorbing its heat of vaporization.
Finally, however, although the gypsum doesn't burn, it shrinks and cracks when heated in a flame. It is known that this tendency to crack can be countered with appropriate additives, such as fiber, especially glass textile fibers, which hold the structure together, and raw vermiculite, which expands when heated, counteracting the gypsum shrinkage. With this knowledge, a number of gypsum-containing products have been developed in which fire-resistance is critical. Such products include fire doors, for example.
Fire doors may be of either the panel or flush types. They include facings on the two major surface, and the core of the door may either be solid or at least partially hollow. Edge banding is included around the door periphery for aesthetic or structural reasons.
Fire doors, as used in residential, commercial and industrial applications, are typically employed in conjunction with fire walls to provide fire protection between different zones of a structure, and particularly to isolate high fire risk areas of a building from the remainder of the structure, such as the garage of a dwelling from its living quarters. Fire doors usually are not capable of indefinitely withstanding the high temperature conditions of a fire but, rather, are designed to maintain the integrity of the fire wall for a limited time to permit the occupants of a building to escape and to delay the spread of the fire until fire control equipment can be brought to the scene.
Various tests have been devised for fire doors and are based on factors, such as the time that a given door would withstand a certain temperature while maintaining its integrity, and hose stream tests which involve the door's ability to withstand the forces of a high pressure water stream. The American Society for Testing Materials (ASTM) has devised tests to establish fire door standards, and these standards are incorporated into building codes and architectural specifications. One such standard, ASTM Method E 152, requires a door to maintain its integrity for periods ranging up to 1.5 hrs. while withstanding progressively higher temperatures and the erosive effects of a high pressure fire hose at the conclusion of the fire exposure.
Considerations in fire door design, in addition to retarding the advance of a fire, include the cost of raw materials and the cost of fabrication. Furthermore, the weight of the door is important, both from the standpoint of ease in handling and the cost of transportation. The strength of the door is also a significant factor, since fire doors must pass the previously noted water stream tests as well as have the requisite structural strength to withstand normal use and abuse.
Fire-resistant doors have been made in a variety of constructions utilizing a number of different materials, including wood, metal and mineral materials. Early forms of fire doors simply comprised wooden cores faced with metal sheeting. Although wood of ample thickness is an effective fire and heat retardant, doors of such construction tended to be heavy and were expensive to fabricate and transport.
Mineral materials have also been employed in the manufacture of fire doors. The core of a commercial metal fire door principally comprises a composition including mineral fibers and a binder. Such doors suffer, however, from a lack of strength, and handling the friable cores results in the production of irritating dust particles during the manufacturing process.
It has also been proposed to make fire doors wherein the core comprises particles of expanded perlite which are bound together by the use of various hydraulic binders including gypsum, cement and inorganic adhesive material. In order to provide sufficient strength, particularly to withstand handling of the core during manufacture, the core is compressed to compact the mixture to a relatively high density, resulting in a heavy door.
Other fire doors have included conventional gypsum wallboard panels as a core material. However, in order to provide sufficient fire resistance, the thickness required of the wallboard is such as to result in an excessively heavy door. Furthermore, internal structural members such as rails or mullions have been found necessary to support and strengthen wallboard panels. The need for such reinforcing elements increases the cost of materials and assembly of such doors. In addition to the above-mentioned considerations, fire doors must, in order to be commercially acceptable, also have other properties that are related to the manufacture, installation and service of the fire-resistant door.
U.S. Pat. No. 4,159,302 discloses a set gypsum containing composition which is especially useful as the core in a solid core fire door, and U.S. Pat. No. 4,811,538 describes a fire door which is partially hollow but has a core of set gypsum faced with fibrous mats. U.S. Pat. No. 4,748,771 discloses set gypsum-containing edge banding for use in fire doors.
The state of the art edge banding described in U.S. Pat. No. 4,748,771 is of tripartite construction, in that it includes in lamination, an inner strip comprising a cast gypsum mixture, an intermediate fiber-reinforced plastic strip, and an outer strip of natural wood. Such edge banding is surprisingly complex and correspondingly expensive. The complexity is necessary, at least in part, because the combination of the gypsum and wood strips alone does not provide the screw-holding capacity required for hinges, latch mechanism, etc.; the thin plastic strip is necessary solely for that reason. The gypsum strip included in the edge banding includes gypsum, glass fiber, raw vermiculite, and clay, together with a small amount of paper fiber (less than 2% by weight), wood chips, and a resin binder, which may be polyvinyl acetate.
In summary, the available gypsum building materials which have the superior mechanical and fire-resistant properties required in technically advanced products such as fire doors often require expensive and potentially hazardous additives, such as asbestos, to achieve those properties. Thus, it is an object of the present invention to provide novel gypsum compositions including safe and inexpensive components from which superior building materials can be made. It is another objective of the invention to provide a process for making such superior building materials from the novel compositions. Yet another objective is the provision of gypsum building materials having superior mechanical and fire-resistant properties. It is still another objective to provide novel fire doors which incorporate the novel building materials.